This invention relates generally to the field of testing electronic circuit boards. More particularly, this invention relates to a method and apparatus especially useful for coupon testing low value components on electronic circuit boards.
Testing high frequency, low value components on a printed circuit board presents many challenges. In the production of circuit boards for radio frequency (RF) and very high speed data applications, manufacturers must often use very small value components that match the frequency requirements of the board design. Since small value parts (e.g., 1 pF capacitors, 10 nH inductors) are generally too physically small to include any visible markings, it is very difficult to determine if a wrong reel was placed into the pick-and-place machine. As a result, a whole production run of boards may be populated with the wrong part, forcing costly repairs and creating significant time-to-market problems.
In-Circuit Testing (ICT) is the traditional mechanism to test component values in order to find such a wrong part. ICT equipment is commercially available from a number of manufacturers such as the Agilent 3070 from Agilent Technologies, 395 Page Mill Road, Palo Alto, Calif. 94306, the GenRad TestStation from GenRad, Inc., 7 Technology Park Drive, Westford, Mass. 01886, and the Teradyne Spectrum from Teradyne, Inc., 321 Harrison Avenue, Boston, Mass. 02118. Current ICT equipment very capably handles standard analog components (capacitors greater than 10 pF; inductors greater than 25 uH) loaded on a printed circuit board, and in isolation can often measure even lower value capacitors with adequate accuracy. However, due to the level of impedance and noise between the signal generator, the probes and the board, smaller component values often cannot be accurately measured. Furthermore, the multiplexing of multiple probes to a single input to the ICT equipment to make measurements can add significant errors to the measurement capabilities on small value inductors and capacitors due to the variation in distance to the ICT instrument and the unloaded impedance at each probe. Even if these components are placed on a xe2x80x9ccouponxe2x80x9d outside of the circuit, current ICT equipment has difficulty measuring such components with accuracy. Specifically, inductors smaller than about 25 uH are generally not testable. Furthermore, though ICT equipment can measure capacitors between 1 pF and 10 pF, the measurement can be noisy and inconsistent.
Another type of test device uses so-called xe2x80x9cflying probesxe2x80x9d that use a set of moveable probes that move in the X and Y directions across a printed circuit board to take measurements at any given location on the circuit board. Such devices are currently available commercially, for example, as the Teradyne Javelin, and the GenRad GR Pilot. Currently, the use of flying probers is mainly restricted to prototyping since they are too slow for high speed, high volume manufacturing applications. Moreover, the cost of accurate X-Y registration is comparatively high and the accuracy limited as a tradeoff for the ability to probe any point on the circuit board. Currently, the measurement accuracy of flying probers is comparable to standard ICT measurements. Thus, the measurement accuracy is often insufficient for high frequency, low value components.
With standard analog components, Manual Visual Inspection (MVI) is often a viable way to verify that a correct component has been loaded on the board. Such visual inspection can often quickly detect when an incorrect part has been loaded due to, for example, incorrect loading of a reel of parts on an automated component insertion machine. Unfortunately, small value components such as those commonly used in Radio Frequency (RF) circuits are generally so physically small that they are not labeled in any manner whatsoever. Therefore, a 5 pF capacitor looks exactly like a 1 pF capacitor, and visual inspection often cannot detect an erroneous component placement.
Automated optical inspection (AOI) equipment, such as the Agilent BV3000, Teradyne Optima 7300 and CR Technology RTI6520 from CR Technology, Inc., a subsidiary of Photon Dynamics, Inc., 6325 San Ignacio, San Jose, Calif. 95119 suffer from the same limitations as MVI. Since RF components are often not marked, these machines cannot determine that wrong parts have been installed.
Reel testers, such as those found in auto insert machines, can verify that the proper reel is loaded by scanning the manufacturer""s barcode on the reel. In the past, there have also been other reel testers which visually inspect the components or verify the component values on the reel. The barcode scanning reel tester scans the barcode placed on the reel from the part manufacturer. This method does not actually verify the values of the components, only the labeling of the reel. If the reel is mislabeled, (a situation that has occurred in the past) thousands of erroneous parts may be installed before the problem is detected.
Visual inspection using a reel tester is not useful for components that cannot be uniquely visually distinguished from other components.
Electrical measurement reel testers were available for axial lead components, where the device could drag probes over component leads to measure. It is not believed that this type of reel tester has been adapted to use with surface mount (SMT) components. The electrical measurement reel tester also has some limitations. First, its measurement accuracy may not be sufficient to accurately test small value inductors and capacitors. Second, a reel tester does not test whether the actual parts loaded onto the system are in the correct location. A programming error on the reel tester could easily translate to mis-loaded parts placed on the boards.
Automated x-ray inspection (AXI) equipment such as the Agilent 5DX, GenRad MV-6100 and CR Technology AXI are designed primarily to find solder defects. An AXI machine cannot measure any component values whatsoever. As a defect solder analysis machine, it adds no value in testing RF inductors and capacitors. However, it may be able to read some part markings and find some orientation faults in polarized capacitors.
Functional testers generally perform tests that simulate the function of a particular circuit implemented on the circuit board. Often, functional testers would be able to catch a mis-loaded part placement on a board, provided the functional test was exhaustive enough to find a particular error. However, due to the time required to perform exhaustive functional tests, thousands of boards could have been mis-loaded before a problem is detected. The expense in repairing these boards is prohibitively high. Moreover, certain circuit components (e.g., bypass capacitors) could be incorrect without being detected by most functional testers. Other component errors could make a circuit""s performance marginal without outright failure in a functional test environment but lead to failures in the field. Additionally, exhaustive testing may be prohibitively complex and time consuming in highly complex circuits, resulting in use of a less than completely exhaustive functional test program that might not identify an erroneous part.
Accordingly, there is currently no known acceptable way to reliably test for assembly errors in circuit boards carrying small valued parts.
The present invention relates generally to coupon test method and apparatus. Objects, advantages and features of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the invention.
In one embodiment consistent with the present invention, a circuit board coupon testing method and apparatus is provided in which a coupon tester uses a linear actuator to carry a test head (a test fixture) and probes for an LCR meter. The linear actuator accurately steps the probes over a coupon of components arranged linearly (preferably) adjacent an edge of the circuit board to measure the parameters of the component. The coupon tester can be integrated with an in-circuit tester to provide further functionality, with the coupon test being carried out simultaneously with a portion of the in-circuit test such as an unpowered portion of the in-circuit test.
A circuit board coupon tester for testing a coupon forming a part of a circuit board consistent with an embodiment of the present invention includes a test fixture for holding the circuit board in a prescribed position. A probe head has a probe for probing a coupon component terminal that forms a part of the circuit board coupon. A linear actuator is connected to the probe head. An LCR meter is electrically coupled to the probe head. A test controller operates under program control to carry out a coupon test by: causing the linear actuator to move the probe head in a linear direction to a desired position; causing the probe to engage the coupon component terminal; and receiving a value associated with the coupon component from the LCR meter.
A circuit board coupon tester, consistent with another embodiment, for testing a coupon forming a part of a circuit board, has a test fixture for holding the circuit board in a prescribed position. A probe head with a probe is used for probing a plurality of coupon component terminals forming a part of the circuit board coupon. A linear actuator is connected to the probe head. An LCR meter is electrically coupled to the probe head. A test controller operates under program control to carry out a coupon test by: causing the linear actuator to move the probe head in a linear direction to a desired position over each component of the coupon; causing the probe to engage each of the plurality of coupon component terminals; and receiving a value associated with each of the plurality of coupon components from the LCR meter.
In another embodiment consistent with the present invention, a method of testing a circuit board, the circuit board having a coupon thereon, the coupon having a plurality of components with a plurality of test pads coupled to each of the plurality of components, the coupon components being arranged linearly adjacent an edge of the circuit board includes holding the circuit board in a fixture in a prescribed position, the fixture having a linear actuator thereon carrying a test head with a probe; under program control, sequentially moving the test head linearly to a position above each of the plurality components; engaging the probe with the test pads for each of the plurality of components; and measuring a value associated with each of the plurality of components.
Another method of testing a circuit board, consistent with certain embodiments of the invention includes providing a circuit board fixture holding the circuit board in a prescribed position; conducting a coupon test on the circuit board; conducting an in-circuit test on the circuit board; and wherein at least a portion of the coupon test is conducted simultaneously with at least a portion of the in-circuit test.
A circuit board panel consistent with certain embodiments of the invention includes at least one circuit board that carries out an electronic function. A test coupon region is preferably situated adjacent an edge of the circuit board panel with a plurality of components, each having a coupon component terminal, arranged linearly along the coupon region. Each of the coupon components have a test pad electrically coupled with the coupon component terminal, wherein the coupon components are arranged along a line approximately parallel with the edge of the circuit board panel and spaced at regular intervals, and wherein at least one of the coupon components is selected from a group consisting of a capacitor having value less than or equal to 10 pf and an inductor having value less than or equal to 25 microHenrys, i.e., small value RF components.
Many variations, equivalents and permutations of these illustrative exemplary embodiments of the invention will occur to those skilled in the art upon consideration of the description that follows. The particular examples above should not be considered to define the scope of the invention.